Summary Cyclin D1 elicits transcriptional effects through inactivation of the retinoblastoma protein and direct association with transcriptional regulators. The current work reveals a molecular relationship between cyclin D1/CDK4 kinase and protein arginine methyltransferase 5 (PRMT5), an enzyme associated with histone methylation and transcriptional repression. Primary tumors of a mouse lymphoma model exhibit increased PRMT5 methyltransferase activity and histone arginine methylation. Analyses demonstrate that MEP50, a PRMT5 co-regulatory factor, is a CDK4 substrate, and phosphorylation increases PRMT5/MEP50 activity. Increased PRMT5 activity mediates key events associated with cyclin D1-dependent neoplastic growth including CUL4 repression, CDT1 overexpression, and DNA re-replication. Importantly, human cancers harboring mutations in Fbx4, the cyclin D1 E3 ligase, exhibit nuclear cyclin D1 accumulation and increased PRMT5 activity.

Deregulation of cyclin D1 occurs in numerous human cancers through mutations, alternative splicing, and gene amplification. Although cancer-derived cyclin D1 mutants are potent oncogenes in vitro and in vivo, the mechanisms whereby they contribute to neoplasia are poorly understood. We now provide evidence derived from both mouse models and human cancer-derived cells revealing that nuclear accumulation of catalytically active mutant cyclin D1/CDK4 complexes triggers DNA rereplication, resulting from Cdt1 stabilization, which in turn triggers the DNA damage checkpoint and p53-dependent apoptosis. Loss of p53 through mutations or targeted deletion results in increased genomic instability and neoplastic growth. Collectively, the data presented reveal mechanistic insights into how uncoupling of critical cell cycle regulatory events will perturb DNA replication fidelity, thereby contributing to neoplastic transformation.[Keywords: Cdt1; Cul4; cyclin D1T286A; cyclin D1P287A; rereplication; p53; genomic instability] Supplemental material is available at http://www.genesdev.org. Received June 22, 2007; revised version accepted September 21, 2007. Dysregulation of the pathways that contribute to the normal integration of growth factor signaling and cell division lies at the heart of neoplastic growth. Cyclin D expression, accumulation, and its ability to associate with catalytic partners CDK4/6 are regulated by mitogenic signaling, allowing D cyclins to serve as mediators of growth factor signaling and cell cycle progression (Gladden and Diehl 2005). Cyclin D1 gene expression depends on activation of the small GTP-binding protein, Ras (Marshall 1999). Ras activates canonical MAP kinase signaling (RAF-MEK-ERK); activation of this pathway is both necessary and sufficient for cyclin D1 expression and association with CDK4 (Albanese et al. 1995;Lavoie et al. 1996;Aktas et al. 1997;Cheng et al. 1998). Ras also contributes to the accumulation of the cyclin D1 protein, via a pathway involving PI3K and Akt, which attenuates the GSK3␤ kinase (Franke et al. 1995(Franke et al. , 1997; GSK3␤-dependent phosphorylation of cyclin D1 on Thr286 promotes cyclin D1 proteolysis (Diehl et al. 1998). Cyclin D1 subcellular localization is also regulated during cell cycle progression. Phosphorylation of cyclin D1 on Thr286 at the G1/S-phase boundary promotes CRM1-dependent nuclear export (Alt et al. 2000). Thus, Thr286 phosphorylation coordinates two critical regulatory events. First, it removes the cyclin D1 kinase from the nucleus during S phase, thereby sequestering it from potential substrates. Second, ubiquitin-mediated destruction of cyclin D1 requires cytoplasmic localization, as the D1 E3 ligase is restricted to the cytoplasm (Lin et al. 2006).Cyclin D1 overexpression occurs in carcinomas of the esophagus, head and neck, breast, and colon, as well as certain B-cell lymphomas (Bartkova et al. 1994a(Bartkova et al. ,b, 1995Gillett et al. 1994;Herman et al. 1995;Barnes and Gillett 1998;Hibberts et al. 1999;Hosokawa et al. 1999; BaniHani...

Mantle cell lymphoma (MCL) is a B-cell lymphoma characterized by overexpression of cyclin D1 due to the t(11;14) chromosomal translocation. While expression of cyclin D1 correlates with MCL development, expression of wild-type (WT) cyclin D1 transgene in murine lymphocytes is unable to drive B-cell lymphoma. As cyclin D1 mutants that are refractory to nuclear export display heighten oncogenicity in vitro compared with WT D1, we generated mice expressing FLAG-D1/T286A, a constitutively nuclear mutant, under the control of the immunoglobulin enhancer, El. D1/T286A transgenic mice universally develop a mature B-cell lymphoma. Expression of D1/T286A in B lymphocytes results in S phase entry in resting lymphocytes and increased apoptosis in spleens of young premalignant mice. Lymphoma onset correlates with perturbations in p53/ MDM2/p19 Arf expression and with BcL-2 overexpression suggesting that alterations in one or both of these pathways may contribute to lymphoma development. Our results describe a cyclin D1-driven model of B-cell lymphomagenesis and provide evidence that nuclearretention of cyclin D1 is oncogenic in vivo.